The use of hot gas generators which produce corrosive or abrasive products of combustion have precluded the use of gas turbines in a direct flow path of combustion products. In a prior art heat generating cycle, a heat recovery steam generator was used in combination with a steam turbine and electrical generator to recover valuable heat which would otherwise be wasted. An air turbine in combination with a gas to air heat recovery heat exchanger provides a means for utilizing what would be an otherwise wasted hot exhaust gas stream.
In U.S. patent application Ser. No. 747,552; filed June 21, 1985 for an Air Cycle Thermodynamic Conversion System, and assigned to the assignee of the present invention, the efficiency of a so-called air bottoming cycle (ABC) was demonstrated and fully explained. To summarize the disclosure of that patent application, an air turbine drives a string of compressors which have intercoolers so that each stage output is cooled to about ambient before being sent on to the next stage. The intercooling process between compressors lowers the work input into the compressor necessary for providing the pressure rise. Moreover, as the compressed air leaves the final compressor stage, it is at a lower temperature so that the heat recovery from the gas turbine exhaust stack flow, with which it is in a heat exchange process, is also greater.
In U.S. patent application Ser. No. 552,213, having the same inventors as the present application and a common assignee, and filed on the same date: a coal gasification plant is shown wherein a gas turbine exhaust provides heat to an air bottoming cycle air turbine. The air turbine, in turn, drives a string of compressors to provide air to the air turbine and air for an oxygen plant which provides oxygen to a coal gasification plant. The coal gasification plant thereafter provides fuel to the gas turbine.
In some processes, such as are used in the manufacture of sulfuric acid, the hot gas products of a sulfur furnace must be cooled but cannot be expanded directly in a gas turbine because of the corrosive nature of the product gas. However, the process of producing sulfur dioxide gas requires a compressed air supply and therefore it would be desirable to provide a gas turbine/compressor package which could somehow use the heat produced in the sulfur burning furnace. In the prior art, the process included the provision of a heat recovery steam generator to absorb heat from the sulfur furnace hot gas stream and to produce steam for a steam turbine. The steam turbine would drive an electric generator to produce electricity some of which would be used to drive an electric motor and the remainder available to sell as power. The electric motor would, in turn, drive a compressor to supply compressed air to the sulfur burning furnace.